This invention relates generally to processes for producing acrolein and acrylic acid, particularly by vapor-phase catalytic oxidation of propylene with air or a gas containing molecular oxygen. More specifically, the invention relates to a process for producing principally acrolein and a by-product quantity of acrylic acid by catalytic oxidation of propylene at an elevated temperature with air or a gas containing molecular oxygen and with the use of a molybdenum-bismuth, multiple-component catalyst.
This oxidation reaction is accompanied by a great generation of heat and is ordinarily carried out in a multitubular, fixed-bed reaction vessel or a fluidized-bed reaction vessel. This invention relates to the use of a multitubular, fixed-bed reaction vessel.
The vapor-phase catalytic oxidation of propylene is already being industrially practiced as a reaction with the object of producing acrolein or as a first-stage reaction of a two-stage oxidation with the object of producing acrylic acid.
In the industrial practice of this oxidation reaction, a catalyst of high activity for conversion of propylene, and, moreover, a catalyst of high selectivity with respect to acrolein or with respect to acrolein and acrylic acid is required. For this reason, a great number of catalysts for this purpose have been proposed. The catalytic performances of many of these catalysts can be considered to be amply high with respect to the yield of the objective product. For example, the catalysts of Japanese Patent Publication Nos. 17711/1972, 27490/1972, 41329/1972, 42241/1972, 42813/1972, 1645/1973, 4763/1973, 4764/1973, 4765/1973, and others are said to afford the objective product or products in high yields of the order of 90 percent or higher as total yields of acrolein and acrylic acid.
However, in the case where the production of acrolein or acrolein and acrylic acid is to be practiced industrially by using these catalysts, various difficulties are encountered with respect to the realization of industrial requirements other than the yield of the objective product or products.
One difficulty arises from the industrial necessity of raising the productivity of the objective product(s). If, for this purpose, the partial pressure of the propylene in the starting material is carelessly raised, hot spots will easily form at the upstream area of catalyst, whereby there will be the danger of a runaway of the reaction. Furthermore, excessive heat generation will give rise to deterioration of the catalyst and a shortening of the catalyst life.
A simple and convenient measure for preventing the formation of hot spots is the known method of diluting parts of the catalyst of high heat generation with an inactive material. This method is disclosed in Japanese Patent Publication Nos. 9859/1959 and 24403/1968 and Japanese Patent Laid Open Nos. 10614/1972 and 127013/1976.
Another method of elevating the productivity is to increase the space velocity, whereby the reaction temperature becomes high as a natural result. However, this gives rise to a tendency toward lowering of the selectivity of the reaction and, in addition, a shortening of the catalyst life. Still another problem is that, when the temperature becomes high, there is the risk of a spontaneous oxidation reaction of the acrolein in the vapor-phase near the outlet of the reaction tube.
With the aim of solving these problems, we have carried out analytical studies relating to improvements in the characteristics of catalysts and various reactions related thereto.
As a result, we have discovered a highly effective process not seen in the prior art and have succeeded in inventing a technique which is greatly advanced for oxidizing propylene.